What is the Gospel, and why did God make it a requirement to be saved?

The word gospel comes from the Greek word euangelion (𝜀𝛼𝛾𝛾𝜆𝜄𝜊𝜈), and we see it used in (Acts 15:7, Rom 1:1). The gospel is indicated in these verses to be a message to be believed and to be saved by. Rom 15:20 specifically ties Christ being named to the hearers being saved. Don’t miss the fact that Christianity is "Christ"-ianity, and without Christ, there is no good news. But what is Christ saving us from? You may have been asked by an obedient, well-meaning street evangelist or friend the question of, "Do you want to be saved?" 

The short answer is that Christ saves us from God Himself. Because Christ was God, we could make the statement that God saves man from God Himself. Mankind need not fear death, nor earthly peril, nor anything as much as he need fear his inevitable meeting with our Maker (Heb 10:31). To understand why God is going to judge a precious group of billions of people, a people made in His image (Gen 1:27), we need to go back to Genesis. Let’s do that. 

In Genesis 3, we see a major turning point for mankind, a pivotal moment where sin enters the world and enters man. If you have heard the story of the Serpent tempting Eve (just take a look at your iPhone device brand image for a visual – but Apple denies any connection to Genesis), then you are aware that Eve eating the fruit was disobedient to God. Adam soon followed suit, and God curses both of them and the earth. The implication of Genesis 3 events is exegeted for us by the Holy Spirit Himself in Romans 5:12. This is a key fact for you to understand: “Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned—” 

In Genesis 3, sin “entered the world”, and that death has spread to every man and woman that has ever lived after Adam and Eve. Romans 5:16 says that Adam's inaugural sin in the garden, is the "one transgression resulting in condemnation” for all mankind. And there you have it. Man is sinful and to be condemned, and further, no man after Adam and Eve will escape the fact that they are condemned as a result of inheriting the “sin nature”, as theologians call it. This sin nature is expressed succinctly in Romans 8:3, which uses the phrase of “likeness of sinful flesh” (cf. Is 53:6, Ps 51:5, Rom 7:14)

We have tied condemnation of man to his inherited sin nature. Because of his inherited sin nature, the Bible tells us that man cannot please God (Rom 8:7-8), nor will man even seek after God in the correct way to be saved (Rom 3:10-11). Further, God cannot overlook sin and must judge sin. We get that from places like Nahum 1:3, Rom 3:7 or 2 Pet 3:9, culminating in the future with the final Great White Throne Judgment in Revelation 20. So now we have tied the inherited sin nature to condemnation and judgment. 

So we should now ask again: what is the Gospel aka good news? The good news is that God sent his Son to be propitiation for our sins (Heb 2:17, Rom 3:25). That is a big word, but it is worth using as it ties us back to the Ark of the Covenant and the Old Testament (OT) sacrificial system. The reason I feel we must do that here is to highlight for you as the reader, that God does not change, whether He is in the OT or He is in the New Testament (NT). God wrote the entire Bible, and if He did change, or if there were contradictions in the text, we would have every right to question whether the perfect, infallible, all powerful God wrote the Bible.

Now, in the OT sacrificial system, once a year on the Day of atonement (Leviticus 16), the high priest sprinkled sacrificial blood on the mercy seat (hilastērion) to satisfy God’s righteous judgment for the sins of the nation. This functioned as: Substitution (the animal dies instead of the people), Propitiation (God’s wrath is satisfied), Atonement (covering), and Reconciliation (God and the people restored). In Romans 3:25, we see the same Greek word hilastērion used to speak of Jesus Christ. The Ark of the Covenant’s mercy seat was the physical place where propitiation happened in the OT, and the NT declares that Jesus Himself has become that mercy seat — the true and final propitiation for sin. Hebrews 10:11-14, which is in the NT and written after the death and resurrection of Christ, says that Christ only needed to offer his life once as a sacrifice to atone for the sin of those who are saved, whereas the OT sacrificial system needed to happen on a recurring basis to be efficacious. The Hebrews text says, "but He [Jesus], having offered one sacrifice for sins for all time."

God has always required payment for sin, and until Christ offered the once and for all sacrifice, God accepted the sacrificial system that He himself had enacted and designed. Even though we live in the "New Covenant" era, a sacrificial requirement is still in place, for God does not change. The only difference for us today is that the sacrifice was made once and for all by Christ. 

You only need believe this. There is no "act" for you to do to gain God's forgiveness. God gets all the credit (Eph 2:8-9) for your salvation.